Machines for producing cross-linked extruded products are generally known. For example, U.S. Pat. No. 4,117,195 discloses a screw extruder into which polymer granules are fed through a feed hopper. The other ingredients, typically cross-linkers, that must be added to the polymer granules are usually injected in liquid form into the lower portion of the feed hopper to the extruder or directly into the neck part of the extruder of this type of machine.
For complete and uniform cross-linking of polymers, it is desirable to achieve adequate mixing of all ingredients. However, this is precisely one of the main disadvantages of the known type of equipment. By merely injecting or dosing certain liquid additive ingredients into a flow of granular polymer material, which is passing through the extruder feed hopper or the neck of the extruder, a high liquid additive concentration is achieved only in the area around the injection site. In these systems, the mixture of liquid additive and granular polymer material must then be homogenized by means of the extruder worm gear with a complicated design as part of the extruder. In addition to the resulting unfavorable conditions for good cross-linking, this requires an extruder with a complex design that can be manufactured only at great expense.
A mixing device for premixing all the ingredients of the reaction mixture, namely the liquid cross-linking additive(s) and the granular polymer material, has been also disclosed in U.S. Pat. No. 5,609,831 issued to Kempter (“the '831 patent”). In the apparatus of the '831 patent, injection or dosing of one or more liquid additives occurs at right angles to the flow of the polymer and results in a preliminary mixing of the ingredients of the reaction mixture by means of the flow effect. The apparatus of the '831 patent, however, requires that the ingredients be further mixed mechanically with the help of a stirring rod. The requirement of mechanical mixing by the stirring rod adds additional complexity, time, and expense to the blending and extrusion process.
In view of at least these deficiencies, a need exists in the industry.